Recently I have been working with Faculty of Engineering at University of Bristol
and BBC Research and Development to shoot some tests for the expanding capabilities of Digital Cinematography. My role on this project was to
oversee the cinematography. In essence we have been trying to calibrate Higher Dynamic
Range, Higher Frame Rate and Higher Resolutions to match the eye/brain pathway
to create highest immersion for the viewing experience. The first tests were shot in November 2012 and since then have been ongoing.
In the human optical system we have a sensitivity of 14 orders of
magnitude of which we can always access 5 orders of magnitude. These 5 orders slide
upwards for bright desert sun and downwards to cope with low light levels of
moon and starlight. The University of Bristol has a display that exhibits this
dynamic range.
Digital Cinematography cameras capture around 12 – 14 stops and 35mm
film captures around 18 stops. Standard displays, TV’s projectors &
computer screens display between 10 and 16 Fstops – that’s between 2 to 3
orders of magnitude of the entire 14 available in the human system, so if
you create a higher dynamic range image of 18 or above stops – it will display beneath
its dynamic range on contemporary displays. The common response from people
seeing this is: “the image looks ‘plastic’”.
The department of Experimental Psychology at Bristol has already undertaken
2D and 3D immersion tests – but these require of 30 minutes of footage. With a
limited budget we decided to shoot the Somerset Carnival because of its high
internal illumination and floats with internal movement. To shoot HDR an Epic
would shoot 50 fps at 4k in HDR mode with a 4 to 6 stop difference giving 18
stops dynamic range.
We then considered mounting 2 Phantoms to shoot 200 fps at 2k in a 3D
mirror rig, one camera exposing the high stop the other the low. Dr Marc Price &
Alia Skeikh of BBC R&D placed two cameras on a rig at BBC London, but found
that artifacts became evident as the tolerances necessary for HDR alignment are
far higher than 3D because you need pixel accurate registration to eliminate these
artifacts.
We calculated on Epic we could record 6 mins of
4k, 50 fps HDR (thats 100 fps) that would take 40 minutes to download. We
decided that due to much higher levels of data output on the Phantom, we would
shoot selected floats at the carnival exhibiting high levels of motion. Had
we shot Phantom in a mirror rig at its highest speed you could easily generate
1 terabyte of data per minute and that would take 6 and half hours to download.
An HDR stop-motion test conducted by Aaron Fang of University of Bristol
Engineering revealed that you need 7 exposures combined to display full higher
dynamic range on the display. So using Red’s strategy of setting a correct
Fstop to build upon for HDR did not exploit the full-potential available.
The hardest thing to expose in cinematography is a subject that emits
light: At our first shoot in Burnham on Sea on the Epic I set the ASA
at 320 at 50 fps with a shutter of 100th, then used a spotmeter to
calculate a stop. But following this through to display, we discovered that a
full HDR image was not achieved using normal cinematographic judgment. I realised that the 100 year old maxim of
exposing to protect highlights was no longer a correct rule for HDR. In fact you
had to expose the ‘correct’ stop ‘virtually’ – What I mean by this is that
if the Fstop should be F5.6 then we would have set the Iris to overexpose three
stops over at F2 plus HDRx highlight protection of three stops under: making 18
stops in total. That might seem obvious now, but on the shoot, sphincters
tightened, because the Epic images looked terribly overexposed.
We had planned a second shoot at Wells and collected material at 2 stops
and 3 stops over and under e-exposed. In HDR terms that’s between 16 to 18
stops respectively. We are about to shoot more footage for immersion testing at
the newly built lab at Bristol University. It should be remembered that the
first tests worldwide with this expanded digital cinematographic form took
place in the South West of the UK.
From the cinematographers viewpoint, it seems to me that counter-intuitively,
lower lighting levels are where HDR will function with most impact on a photographic
level. Obviously in the highlights colour formation will be held better than in
standard systems, but this is a technical issue, rather than specifically an artistic one.
But this is interesting because it may in fact require a renaissance of
cinematic judgment – someone will need to know that the end result will be
fine. With regard low light, the way we read an image may allow the
cinematographer to offer clues when underscoring plot, story & emotional
cues, in a far more subtle way than with standard dynamic range that the cinematographic
arts have used for decades: that of trying to represent 5 orders of magnitude
in a 2 – 3 orders of magnitude display.
